Determination of the degree of cross-linking of phenol-formaldehyde fibres

the organic solvent, and in the anomalous behavior of the extinction coefficients of these bands on dilution with carbon tetrachloride (see Fig. 2). This dependence can be used to determine the water content of NMP.

The procedures described are recommended for determining the water content of organic solvents; however the coefficients m and n:and the shape of the calibration curves should be refined in conformity with the special features of the spectrophotometers used.

The fibres which are prepared from a phenol-formaldehyde resin (PFR) melt acquire a complex of textile properties after hardening, as a result of which they become infusible due to cross-linking of the polymer macromolecules [i]. The degree of cross-linking depends on the process parameters, mainly on temperature, and determines not only the quality of the fibre, but also its ability to undergo subsequent heat-treatment. In this connection, a quantitative evaluation of the degree of cross-linking is a technologicaly important factor.

For the indicated purpose, the authors have used the method of pyrolytic gas chromato- graphy, assuming that the amount of monomer evolved during the degradation process will be sharply reduced with increase in the degree of cross-linking of the polymer macromolecules.

As objects of study we took phenol-formaldehyde fibres which had been hardened by heat- ing to an assigned temperature in an 18.5% aqueous solution of formaldehyde containing hy- drochloric acid, with subsequent washing and drying.

Pyrolysis of the fibre was carried out in a pyrolyzer of the tube type, in the temperature range 200-550C. Separation and analysis of the pyrolysis products evolved was carried out on a "Tsvet-104" chromatograph using a column of "Chromaton-N-AW-HMD" having 15% Apiezon by weight.

TABLE i. Effect of Hardening Conditions on the Ratio of the Areas of Characteristic Peaks

It was found that the characteristic peaks for all the investigated fibre samples were peaks for phenol and water, As the criterion for evaluating the degree of cross-linking, we selected the ratio of the areas of these peaks, Sp/S w. This ratio does not depend on the sample weight and is a more reliable evaluation criterion than the area of the phenol peak.

As is evident from Table i, with increase in the hardening temperature of the fibre, the temperature for appearance of the phenol peak rises, and the Sp/S w ratio decreases sharply. It is obvious that on increasing the degree of cross-linking, the degree of polymer degradation during the pyrolysis process decreases and the rate of the inter- and intramolecular dehy- dration reactions increases. In the pyrolysis of the fibre which has been heated at I05C for a long time, essentially no phenol is evolved, which may be connected up with a maximum degree of cross-linking of the macromolecules. Data from optical microscopy confirm this conclusion: Fibre treated in this way has practically no core.

Consequently, the method of pyrolytic gas chromatography is sensitive to the degree of cross-linking of PFR. The authors suggest that this method can be used for determining the degree of cross-linking of polymers.

i.

LITERATURE CITED

T. N. Koziorova, et al., Khim. Volokna, No. 2, 18 (1975).

POLAROGRAPHIC DETERMINATION OF THE IRON ION CONTENT

OF DIMETHYLFORMAMIDE OR DIMETHYLACETAMIDE MEDIA

L. N. Bykova, O. Ya. Chesnokova, and V. V. Bogoslovskii

UDC 677.021.12:543.253

Aprotic dipolar solvents -- dimethylformamide (DMF) and dimethylacetamide (DMAc) -- have found wide application in the manufacture of man-made fibres. The presence of iron salts in technological solutions based on DMF or DMAc is undesirable, since the iron ions may accelerate the process of polymer degradation and affect the physicomechanical properties of the fibres obtained [i, p. 83]. In this connection is necessary to develop a rapid method of monitoring the iron ion content of regenerated solvents which will make i tpossible to eliminate the return of solvent having an elevated iron salts content to the technological process.

The regenerated solvents are colored to various degrees; therefore the preparation of samples for analysis using the photometric method requires considerable time [2]. A polar- ographic method for direct determination of iron ion content will make it possible to cut down the analysis time from 1.5-2 h to about 20 min.

In the present communication we examine the features of the polarographic reduction of trivalent iron ions at a dropping mercury electrode (DME) in a medium of DMF, DMAc, or mix-

3+ tures of these with water, and procedures are given for determining the Fe ion content in these solvents.

Our studies were carried out on a PPT-I polarograph under a variable-current regime with the application of a trapezoidal s igna l . The rate of potential scanning was 5 mV/sec. The polarograms were taken in a three-electrode cell, relative to a silver electrode. A mercury pool served as the auxiliary electrode. We used a dropping mercury electrode with a drop time of 3.2 sec in a 0.I M solution of LiCIO~ in DMF, without application of a voltage. Oxygen was removed from the solutions by a current of specially purified nitrogen. To prepare model mixtures, we used DMF and DMAc which had been stored over freshly ignited A1203 and distilled under ~acuum. The purity of the solvents was checked polarographically. The DMF contained formic acid in the amount of less than 0.0004 g-moles/liter; and theDMAc contained acetic acid in an amount less than 0.0003 g-mole/liter. The water content of the solvents, determined by use of the Karl Fisher reagent, was 0.04-0.20% by wt. To construct calibration